The costs and benfits of chemical defense in a specialist insect herbivore

• Applicant: Dr. Franziska Beran
• Subject Area: Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 501638262

Many herbivorous insects protect themselves from predators by accumulating chemical defense compounds of their food plants. However, this accumulation of toxic compounds in the insect can also have physiological costs, a phenomenon that has rarely been studied so far. This project aims to characterize the costs and benefits of this defense strategy, using the horseradish flea beetle (Phyllotreta armoraciae) as a model. The host plants of this beetle contain mustard oil glucosides, which are hydrolyzed upon herbivory by the plant enzyme myrosinase to form toxic mustard oils that deter non-adapted herbivores. Horseradish flea beetles can at least partially prevent this hydrolysis, and instead accumulate high amounts of these substances in their bodies. The stored mustard oil glucosides can in turn be converted into toxic mustard oils by the beetle's own myrosinases. Genetic studies have revealed that the number of myrosinase genes encoded in the beetle genome varies between individuals, which influences their level of myrosinase activity. However, the consequences of this genetic variability on insect fitness (= costs) and resistance to antagonists (= benefits) are unknown. To fill this knowledge gap, we will knock out the different myrosinase genes individually and in combination using CRISPR/Cas9 in our established laboratory population and then compare the fitness and resistance of the different knock-out mutants with those of the wild type. In addition, we will examine laboratory-bred beetle lines that differ from each other regarding the number of encoded myrosinase genes and the levels of myrosinase activity. Specifically, we will compare the performance, developmental time, and reproductive rates of the different beetle genotypes, and evaluate their resistance to entomopathogenic fungi and entomopathogenic nematodes. In addition to characterizing laboratory populations under controlled conditions, the chemical defense of natural beetle populations will be investigated. The focus will be on the influence of copy number variation of myrosinase genes on chemical defense in natural beetle populations and how this influences the interaction with antagonists. With this work, we aim to contribute to a better understanding of the ecological significance of genetic variability in natural populations. In addition, our results can be used to develop sustainable control strategies against Phyllotreta flea beetle pests using entomopathogenic fungi and entomopathogenic nematodes.

• DFG Programme: Research Grants